Minimizing ringing in wide band gap semiconductor devices

ABSTRACT

Embodiments include a power conversion circuit comprising first and second semiconductor switches, and a drive circuit configured to create a period of operational overlap for the first and second switches by setting a gate voltage of the first switch to an intermediate value above a threshold voltage of the first switch, during turn-on and turn-off operations of the second switch. Embodiments also include a method of operating first and second semiconductor devices, comprising: reducing a gate voltage of the first device to an intermediate value above a threshold voltage while the second device is off; turning off the first device after the second device is on; increasing the gate voltage of the first device to the intermediate value while the second device is on; and fully turning on the first device after the second device is off.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/494,359 filed Apr. 21, 2017, and will issue as U.S. Pat. No. 10,193,544 on Jan. 29, 2019, which is herein incorporated by reference in its entirety.

TECHNICAL FIELD

This application generally relates to power electronics and more specifically, to minimizing ringing in power semiconductor devices.

BACKGROUND

The material properties of wide band gap (WBG) semiconductors, such as, for example, silicon carbide (SiC) or gallium nitride (GaN), permit operation at much higher voltages, frequencies, and temperatures than conventional semiconductors, including those made of silicon (Si) or gallium arsenide (GaAs). These features can lead to smaller and more energy-efficient circuits. Recently, WBG semiconductor devices are increasingly being used in high power applications, like high-speed switching for power modules and in charging modules for hybrid and all-electric vehicles.

One challenge facing most applications of WBG devices, including hard-switched power converter or inverter applications, is the occurrence of high frequency (e.g., greater than 30 megahertz (MHz)) ringing, or oscillations, during switching. This high frequency ringing induces electromagnetic interference (EMI) noises to surrounding circuitry (such as, e.g., control lines and measurement lines), as well as other sub-system components, thereby affecting overall system performance. The ringing is primarily caused by high voltage (dv/dt) and current (di/dt) transients induced by the WBG device during switching, which excites parasitic inductance (L) and capacitance (C) in the circuit, thereby causing the device to oscillate during switching.

Existing solutions for minimizing parasitic inductance in WBG devices include improving a packaging of the devices, for example, by reducing stray inductance resulting from the packaging. However, this solution can be expensive and difficult to achieve, especially in power modules rated for more than 300 amperes (A) and designed for use in hybrid and electric vehicles. Also, reducing packaging stray inductance does not eliminate the ringing that occurs during switching. Another existing solution attempts to minimize the ringing by adding external passive components, such as R/C (snubber circuits), to absorb the ringing energy. However, this solution requires the use of additional components, increases packaging cost and size, and reduces reliability during high temperature operation. In addition, the introduction of additional resistors and other external passive components reduces the device dv/dt, di/dt speeds, which in turn greatly increases the switching loss of the device. Another downside of these and other similar existing solutions is that they require external modifications to the WBG device that cannot be controlled or adjusted to manipulate the amount of ringing.

Accordingly, there is still a need in the art for techniques to minimizing ringing or oscillations in wide band gap semiconductor devices during high speed switching.

SUMMARY

The invention is intended to solve the above-noted and other problems by providing gate modulation techniques configured to create a period of operational overlap while switching between first and second semiconductor devices, the period of overlap minimizing the ringing that typically arises during switching by reducing lumped stray capacitance and increasing lumped loop resistance in the circuit. In addition, the techniques described herein utilize inherent characteristics of the semiconductor devices to control ringing and therefore, require no additional hardware and can be adjusted (e.g., enabled or disabled) as needed for a given practical application.

For example, one embodiment includes a power conversion circuit comprising first and second semiconductor switches, and a drive circuit configured to create a period of operational overlap for the first and second switches by setting a gate voltage of the first switch to an intermediate value above a threshold voltage of the first switch, during turn-on and turn-off operations of the second switch.

Another example embodiment includes a method of operating first and second semiconductor devices, comprising: reducing a gate voltage of the first device to an intermediate value above a threshold voltage while the second device is off; turning off the first device after the second device is on; increasing the gate voltage of the first device to the intermediate value while the second device is on; and fully turning on the first device after the second device is off.

As will be appreciated, this disclosure is defined by the appended claims. The description summarizes aspects of the embodiments and should not be used to limit the claims. Other implementations are contemplated in accordance with the techniques described herein, as will be apparent to one having ordinary skill in the art upon examination of the following drawings and detail description, and such implementations are intended to within the scope of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made to embodiments shown in the following drawings. The components in the drawings are not necessarily to scale and related elements may be omitted, or in some instances proportions may have been exaggerated, so as to emphasize and clearly illustrate the novel features described herein. In addition, system components can be variously arranged, as known in the art. Further, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a circuit diagram illustrating an example configuration of a power conversion circuit that can accept application of certain embodiments.

FIG. 2 is a time chart illustrating voltage waveforms during conventional operation of a power conversion circuit.

FIG. 3 is a time chart illustrating example voltage waveforms during application of a gate modulation technique to the power conversion circuit shown in FIG. 1, in accordance with certain embodiments.

FIG. 4 is a flow diagram of an example method of operating first and second semiconductor devices in accordance with certain embodiments.

FIG. 5 is a graph comparing oscillations associated with the conventional circuit operation shown in FIG. 2 to oscillations associated with the gate modulation technique shown in FIG. 3.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

While the invention may be embodied in various forms, there are shown in the drawings, and will hereinafter be described, some exemplary and non-limiting embodiments, with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.

In this application, the use of the disjunctive is intended to include the conjunctive. The use of definite or indefinite articles is not intended to indicate cardinality. In particular, a reference to “the” object or “a” and “an” object is intended to denote also one of a possible plurality of such objects.

FIG. 1 illustrates an exemplary power conversion circuit 100 to which embodiments described herein can be applied. The power conversion circuit 100 represents at least a portion of a power converter for converting DC input voltage to DC output voltage, or a power inverter for converting DC input voltage to AC output voltage. In some cases, the power conversion circuit 100 forms part of a power module that may be included in, for example, an electric vehicle, hybrid electric vehicle (HEV), plug-in electric vehicle (PHEV), or battery electric vehicle (BEV). The power conversion circuit 100 may be included on a single integrated circuit (IC) or may comprise two or more ICs or modules electrically coupled together.

As shown in FIG. 1, the power conversion circuit 100 includes a power semiconductor device 102, or upper switch Q1, and a power semiconductor device 104, or lower switch Q2, connected in series. In the illustrated embodiment, the semiconductor devices 102 and 104 (also referred to herein as “transistors Q1 and Q2”) are n-type metal-oxide semiconductor field-effect transistors (MOSFETs). In other embodiments, the semiconductor devices 102 and 104 may be another type of semiconductor device suitable for high frequency power switching applications (such as, e.g., IGBT). As shown, the semiconductor device 102 is connected in parallel to a power diode 106, and the semiconductor device 104 is connected in parallel to a power diode 108. The power diodes 106 and 108 may be any suitable type of semiconductor diode, such as, for example, a PIN diode, an antiparallel diode, or a Shottky diode. As shown, the semiconductor switch 102 and the power diode 106 form an upper arm of the power conversion circuit 100, while the semiconductor switch 104 and the power diode 108 form a lower arm of the circuit 100. In some cases, the power conversion circuit 100 may be configured such that the upper switch Q1 operates as a passive device, while the lower switch Q2 operates as an active device.

The semiconductor devices 102 and 104 and/or the diodes 106 and 108 can be made from a wide bandgap (WBG) semiconductor material, such as, e.g., silicon carbide (SiC), gallium nitride (GaN), etc. In a preferred embodiment, each of the power switches 102 and 104 and the power diodes 106 and 108 are made of SiC in order to enable operation of the circuit 100 at higher switching frequencies. However, the faster rise and fall times obtained by pairing SiC switches with SiC diodes typically excite oscillations, or ringing, during both turn-on and turn-off operations, especially when using hard switching behavior, and may lead to cross talk and signal distortion.

As will be appreciated, various parasitic components may be unavoidably present in the circuit 100 (also referred to as a “power loop”). In FIG. 1, these parasitic components are illustrated as a lumped loop resistance associated with conductors in the power loop, a lumped loop inductance associated with the wiring or packaging of the power loop, and lumped stray capacitances Q1 and Q2 present in the upper and lower arms, respectively, of the power loop. The stray or parasitic inductive and capacitive components of the power circuit 100 typically give rise to considerable transient effects, including ringing or oscillations during switching.

As shown in FIG. 1, a first terminal of a load inductor 110 is connected between the upper and lower arms of the circuit 100, and a second terminal of the load inductor 110 is connected to the upper arm of the circuit 100. A DC link capacitor 112 is connected to the second terminal of the load inductor 110 and to the lower arm of the circuit 100. In some cases, the DC link capacitor 112 may be coupled in parallel to a DC power source or battery (not shown) to serve as a load-balancing energy storage device, for example, where the power conversion circuit 100 is included in a power inverter for converting the DC power to an AC power output.

As also shown, the power conversion circuit 100 further includes a drive circuit 114 for driving the semiconductor devices 102 and 104. The driver circuit 114 includes a gate driver 116 connected to a gate of the upper transistor Q1, a gate driver 118 connected to a gate of the lower transistor Q2, and a controller 120 configured to provide inputs to the gate drivers 116 and 118 for controlling gate voltages of the transistors Q1 and Q2, respectively. In the illustrated embodiment, the gate drivers 116 and 118 are power amplifiers configured to receive a low-power input from the controller 120 and produce a high-current drive input for the respective gates of the transistors Q1 and Q2. The controller 120 may be any suitable type of integrated circuit (IC) or microcontroller capable of providing an input to the gate drivers 116 and 118, such as, for example, a pulse-width modulation (PWM) controller. The drive circuit 114 may be implemented as a single integrated circuit comprising all of its components, or a collection of separate, dedicated integrated circuits, each comprising one or more components.

In certain cases, the power conversion circuit 100 converts DC power into AC power by performing on-off switching control on the semiconductor devices 102 and 104 using the drive circuit 114. In particular, using the corresponding gate driver 116, 118, the controller 120 controls the level of the gate voltage (Vgs) applied to the gate of each transistor Q1, Q2 to transition the respective semiconductor device 102, 104 between an on-state and an off-state. The level of Vgs required to fully turn on the transistors Q1, Q2 may be referred to herein as a maximum operating value or Vgs_(High) (e.g., 15 volts (V)). The level of Vgs required to turn off the transistors Q1, Q2 may be referred to herein as a minimum operating value or Vgs_(Low) (e.g., 0 V).

In a preferred embodiment, the controller 120 utilizes a hard switching process, wherein the semiconductor devices 102 and 104 exhibit hard turn-on and hard turn-off behavior, for example, through use of a pulse width modulation (PWM) control signal. To achieve hard switching, the switching moment, or the time to transition from on to off or vice versa, is minimized by cutting off the load current within the turn-on or turn-off times. However, shortening the switching moment typically results in a higher frequency of electromagnetic interference (EMI), which can produce a more apparent noise problem.

During conventional operation of the power conversion circuit 100, the controller 120 alternately turns on the upper and lower switches 102, 104 to prevent a through current from flowing through both of the transistors Q1 and Q2 at the same time (also known as a “short-circuit” or “shoot-through” event). Moreover, to ensure that the semiconductor switches 102 and 104 are not inadvertently turned on at the same time (e.g., due to noise in the drive circuit 114 or a sudden change in di/dt) while transitioning from one switch to the other, both of the switches 102 and 104 are typically placed in the off-state for a predetermined period of time, known as “dead time.” This is illustrated by FIG. 2, which is a time chart 200 comprising a first voltage waveform 202 representing gate voltage (Vgs) levels for the upper switch Q1 and a second voltage waveform 204 representing gate voltage (Vgs) levels for the lower switch Q2, during conventional operation of the circuit 100.

As shown in FIG. 2, the controller 120 inserts dead time, T_(Delay), after turning off the upper switch Q1 and before turning on the lower switch Q2, or from time t₁ to t₂. Similarly, though not labeled in FIG. 2, a second dead time is added between turning off the lower switch Q2 and turning on the upper switch Q1, or from time t₃ to t₄. Thus, as shown by the time period, T_(ON), the lower switch Q2 turns on only after the dead time T_(Delay) has passed, and turns off before the second dead time begins. As will be appreciated, the introduction of dead time typically causes an increase in power loss if it is excessively long. Thus, most circuit designers strive to minimize the required amount of dead time.

FIG. 3 is a time chart 300 showing exemplary application of a gate modulation technique to the power conversion circuit 100 to minimize ringing during switching, in accordance with embodiments. In FIG. 3, a voltage waveform 302 represents operation of the upper switch Q1, or the semiconductor device 102 shown in FIG. 1, and a voltage waveform 304 represents operation of the lower switch Q2, or the semiconductor device 104 shown in FIG. 1. In embodiments, the voltage waveforms 302 and 304 may be achieved by using the drive circuit 114, or more specifically, the controller 120 included therein, to control operation of the switches Q1 and Q2 according to a pulse-width modulation (PWM) control signal. Generally, the gate modulation technique alters the conventional turn-on and turn-off operations of the transistors Q1 and Q2 by (1) adding an intermediate operating state between the on- and off-states of the upper switch Q1, wherein the gate voltage of Q1 is set to an intermediate value above a threshold voltage of the upper switch Q1, (2) using the intermediate state to create a period of operational overlap when switching operation between the upper switch Q1 and the lower switch Q2, and (3) virtually eliminating dead time during switching.

As shown in FIG. 3, a first period of operational overlap, T_(Overlap/ON) is associated with a turn-on operation of the lower switch Q2, and a second period of operational overlap, T_(Overlap/OFF), is associated with a turn-off operation of the lower switch Q2. Initially, the upper switch Q1 is in an on-state, wherein the gate voltage of the transistor Q1 is set to Vgs_(High), and the lower switch Q2 is in an off-state, wherein the gate voltage of the transistor Q2 is set to Vgs_(Low). At time t₁, the gate voltage of the upper switch Q1 is reduced or adjusted to an intermediate value, Vgs_(Int), that is below the maximum operating value, Vgs_(High), but above a gate threshold voltage, Vgs_(Threshold). During the transition to the intermediate value, the lower switch Q2 remains in the off-state (e.g., from time t₁ to time t₂).

As will be appreciated, the gate threshold voltage is the minimum amount of charge required at the gate to ready the transistor for carrying current. Once the gate voltage, V_(gs), crosses the threshold value, current starts flowing into the transistor. As a result, the transistor may be considered “on,” in a technical sense, as long as the gate voltage exceeds the threshold voltage (e.g., Vgs>Vgs_(Threshold)). Accordingly, in FIG. 3, during the time interval t₁ to t₃, even though the gate voltage of the upper switch Q1 is reduced below the maximum operating value Vgs_(High), the switch Q1 is still technically on because the gate voltage is above the threshold value.

At time t₂, the lower switch Q2 is turned on by setting or adjusting the gate voltage of the transistor Q2 to the maximum operating value, Vgs_(High). As shown in FIG. 3, this transition is completed while the gate voltage of the upper switch Q1 remains at the intermediate value, Vgs_(Int). Thus, the first period of operational overlap, T_(Overlap/ON), starts at time t₂, once both switches Q1 and Q2 are considered on. As shown in FIG. 3, the period T_(Overlap/ON) ends at time t₃ upon transitioning the upper switch Q1 to the off-state by setting the gate voltage of the transistor Q1 to Vgs_(Low). As shown in FIG. 3, the lower switch Q2 remains in the on-state during this transition. Thus, the upper switch Q1 is in the intermediate state before and after the lower switch Q2 is turned on.

During the period of overlap, current flows through both transistors Q1 and Q2 at the same time, thus causing a short-circuit or shoot-through condition. According to embodiments, the period of overlap can be configured so that a controlled surge in current is momentarily achieved across both transistors Q1 and Q2. The current surge can be controlled in both magnitude and duration by selecting an appropriate intermediate value, Vgs_(Int), for the upper switch Q1 and by selecting an appropriate length of time (e.g, t₂ to t₃) for the period of overlap, T_(Overlap/ON). In embodiments, the intermediate value, Vgs_(Int), can be selected to be just high enough to cause the shoot-through event, but low enough to avoid damage to the power conversion circuit 100. For example, the Vgs_(Int) may be 1-2 volts above the threshold voltage for the upper switch Q1. Likewise, the length of time between t₂ and t₃ can be selected so as to avoid damage to the circuit 100 from the shoot-through condition.

Creation of the shoot-through event during the time period T_(Overlap/ON) momentarily dissipates energy through the upper switch Q1, which lowers the lumped stray capacitance of the upper switch Q1 to a varied or non-existent state and increases the lumped loop resistance of the circuit 100. This reduces or minimizes oscillations that would normally appear during a turn-on operation of the lower switch Q2 and thus, allows the lower switch Q2 to be turned on at a relatively large impedance state.

As shown in FIG. 3, the upper switch Q2 remains fully on for a time period T_(ON) that starts at time t₃, or after the short circuit event is removed, and ends at time t₄. At time t₄, the gate voltage of the upper switch Q1 is increased from the minimum operating value, Vgs_(Low), to the intermediate value, Vgs_(Int), while the lower switch Q2 remains in the on-state. Thus, from time t₄ to time t₅, a second period of operational overlap, T_(Overlap/OFF), is created by allowing current to pass through both transistors Q1 and Q2 at the same time.

Like the first period of overlap, the period T_(Overlap/OFF) causes a controlled surge of current that can be controlled in magnitude and duration. Also like the first period of overlap, creation of the shoot-through event during the time period T_(Overlap/OFF) momentarily dissipates energy through the upper switch Q1, which lowers the lumped stray capacitance of the upper switch Q1 to a varied or non-existent state and increases the lumped loop resistance of the circuit 100. This reduces or minimizes oscillations that would normally appear during a turn-off operation of the lower switch Q2 and thus, allows the lower switch Q2 to be turned off at a relatively low impedance state.

As shown in FIG. 3, the second period of overlap T_(Overlap/OFF) ends at time is upon turning off the lower switch Q2 by reducing the gate voltage of the transistor Q2 to the minimum operating value, Vgs_(Low). At time t6, the upper switch Q1 is turned on fully by increasing the gate voltage from the intermediate value Vgs_(Int) to the maximum operating value Vgs_(High). As shown in FIG. 3, the lower switch Q2 remains off during this transition. Thus, the upper switch Q1 is in the intermediate state before and after turning off the lower switch Q2.

FIG. 4 illustrates a flow diagram of an example method 400 of operating first and second semiconductor switches in a power conversion circuit 100, such as, e.g., the power conversion circuit 100 shown in FIG. 1, in accordance with embodiments. The method 400 may include, or may be similar to, the gate modulation technique shown in FIG. 3 and described herein. The method 400 may be carried out by one or more processors (or controllers) included in the power conversion circuit. In one embodiment, the method 400 is implemented, at least in part, by the controller 120 of the drive circuit 114 executing software stored in a memory (not shown) of the drive circuit 114, and interacting with one or more components of the driver circuit 114, such as, for example, the gate drivers 116 and 118, and/or the power conversion circuit 100.

The method 400 begins at step 402, where a gate voltage of a first switch (e.g., upper switch Q1 of FIG. 3) is reduced to an intermediate value (e.g., Vgs_(Int)) above a threshold value (Vgs_(Threshold)) associated with the first switch, while a second switch (e.g., lower switch Q2 of FIG. 3) is in an off-state. At step 404, the second switch is turned on, or transitioned to an on-state, by increasing a gate voltage of the second switch from a minimum operating value, Vgs_(Low), to a maximum operating value, Vgs_(High). At this point, a first period of operational overlap begins as both the first switch and the second switch are technically on. During the period of overlap, a short circuit is formed across the two switches, thus delivering a controlled surge of current to the transistors Q1 and Q2. At step 406, while the second switch is on, the first switch is turned off by decreasing the gate voltage of the first switch from the intermediate value, Vgs_(Int), to the minimum operating value, Vgs_(Low). Thus, the first period of operational overlap ends at step 406, at which point the second switch continues to operate in the on-state.

The method 400 further includes step 408, where the gate voltage of the first switch is increased from the minimum operating value, Vgs_(Low), to the intermediate value, Vgs_(Int), while the second switch remains in the on-state. This initiates a second period of operational overlap. At step 410, the second switch is turned off, or transitioned to an off-state, by decreasing the gate voltage of the second switch from the maximum operating value, Vgs_(High), to the minimum operating value Vgs_(Low), thus ending the second period of overlap. At step 412, while the second switch is turned off, the first switch is fully turned on by increasing the gate voltage of the first switch from the intermediate value, Vgs_(Int) to the maximum operating value, Vgs_(High). Thus, the second period of operational overlap beings at step 408, while both transistors are technically on, and ends at step 410, once the second switch is no longer on and before the first switch is fully on.

FIG. 5 shows a graph illustrating a comparison between oscillations produced during the conventional circuit operation shown in FIG. 2 and oscillations produced while using the gate modulation technique shown in FIG. 3 and described herein. As shown, conventional operation of SiC switches results in significant ringing or oscillations at initial turn-on, while SiC switches using gate modulation have reduced oscillations at turn-on and faster di/dt.

Thus, unlike conventional system, the gate modulation technique described herein utilizes the inherent properties of semiconductor switches to minimize ringing, or oscillations, rather than relying on additional hardware to achieve this result. In addition, the gate modulation technique can be easily enabled or disabled for a given circuit by simply adjusting the inputs provided by the controller to the first and second switches, thus providing another advantage over conventional systems.

Any process descriptions or blocks in the figures, such as FIG. 4, should be understood as representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process, and alternate implementations are included within the scope of the embodiments described herein, in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those having ordinary skill in the art.

It should be emphasized that the above-described embodiments, particularly, any “preferred” embodiments, are possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) without substantially departing from the spirit and principles of the techniques described herein. All such modifications are intended to be included herein within the scope of this disclosure and protected by the following claims. 

1. A power conversion circuit, comprising: a controller; first and second power amplifiers electrically coupled to the controller; a first semiconductor switch with a first gate electrically coupled to the first power amplifier; a second semiconductor switch with a first gate electrically coupled to the second power amplifier, a current conducting path of the first semiconductor switch being in series with a current conducting path of the second semiconductor switch; and wherein the controller is to, when controlling the second semiconductor switch to turn on and turn off, drive the first power amplifier to create a period of operational overlap for the first and second semiconductor switches by setting a gate voltage of the first semiconductor switch to an intermediate value above a threshold voltage of the first semiconductor switch.
 2. The power conversion circuit of claim 1, wherein to create the period of operational overlap, the controller is to: at a first time, set the gate voltage of the first semiconductor switch to the intermediate value; and at a second time, set the gate voltage of the first semiconductor switch to a low value.
 3. The power conversion circuit of claim 2, wherein to create the period of operational overlap, the controller is to at a third time between the first time and the second time, set the gate voltage of the second semiconductor switch to a high value.
 4. The power conversion circuit of claim 3, wherein a duration between the third time and the second time is selected to avoid damage caused by a short-circuit during the period of operation overlap.
 5. The power conversion circuit of claim 1, wherein the threshold voltage is a minimum amount of charge define by operational characteristics of the first semiconductor switch that is required at the gate to provide the conductive path between the drain and the source.
 6. The power conversion circuit of claim 1, wherein during the period of operational overlap, a short-circuit occurs between a drain of the first semiconductor switch and ground of the power conversion circuit connected to the source of the second semiconductor switch.
 7. The power conversion circuit of claim 6, wherein the intermediate value is set to control a surge current during the short-circuit.
 8. The power conversion circuit of claim 1, wherein the first and second semiconductor switches are metal oxide semiconductor field-effect transistors (MOSFETs) comprising a wide band gap semiconductor material.
 9. The power conversion circuit of claim 8, wherein the wide band gap semiconductor material is Silicon Carbide (SiC).
 10. The power conversion circuit of claim 1, including: a first diode in parallel with a drain and a source of the first semiconductor switch; and a second diode in parallel with a drain and a source of the second semiconductor switch.
 11. The power conversion circuit of claim 10, wherein the first and second diode are made of Silicon Carbide (SiC). 